Femoral slideway and method

ABSTRACT

A femoral slideway and femoral slideway/femur-size template combination is employed to create an installed knee prosthesis that creates a reduced amount of stress in the collateral ligaments relative to previous prosthesis. In one embodiment, the reduced stress is achieved by employing a femoral slideway/femur-size template combination which results in installation of a femoral slideway which has a smaller dorsal-ventral dimension than a corresponding dimension of the femur prior to resection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/953,819, filed on Dec. 10, 2007, which is a continuation of U.S.application Ser. No. 10/616,102, filed on Jul. 9, 2003, now issued asU.S. Pat. No. 7,306,609, which is a divisional of U.S. application Ser.No. 09/517,674, filed on Mar. 2, 2000, now abandoned, which claimspriority under 35 U.S.C. § 119 to German Patent Application No. DE29906909.5, filed on Apr. 16, 1999, and to German Patent Application No.DE 29903766.5, filed on Mar. 2, 1999, the entire contents of eachapplication incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a femoral slideway, and more specifically to afemoral slideway/femur-size template arrangement and a kneeendoprosthesis system with such a femoral slideway.

Description of the Related Art

A femoral slideway of this kind is disclosed, for example, in the Germanpatent DE 40 41 002 C2. In the surgical technique customarily used forknee-joint replacement by means of such a femoral slideway, an equalamount of bone is removed from the two condyles of the femur, so thatthe anterior or ventral cut is parallel to the posterior or dorsal cut.When the implant is in the position thus defined, the axis of rotationof the implant no longer coincides with the axis specified by thearrangement of the collateral ligaments, and this position is notanatomical inasmuch as when flexed, the implant is seated either tootightly on the medial side or too loosely on the lateral side.

The remedy that was recommended some time ago, namely an outwardrotation of the cutting guide such that in the posterior region lessbone is removed laterally than medially, whereas anterior-laterally morebone is removed than on the anterior-medial side, also presentsdisadvantages, which the construction of the femoral slideway proposedin DE 197 16 879 A1 of the applicant is designed to eliminate. The cruxof this solution is to rotate the anterior or ventral cut in thetransverse plane.

However, this more recent solution also requires improvement withrespect to optimizing the joint function in cooperation with thecollateral ligaments in particular, especially in order to reduce theload imposed thereon.

SUMMARY OF THE INVENTION

It is thus the object of the invention to disclose a femoral slidewaywith further improved function, as well as a knee endoprosthesis systemthat can be efficiently constructed and employed and has such a femoralslideway as its essential element, and finally an advantageousarrangement comprising femoral slideway and femur-size template.

This object is achieved in its first, foremost aspect by a femoralslideway with the features described in the present application.

The invention includes the essential idea that it is advantageous toprepare for a knee-joint replacement by resecting more bone from thefemur dorsally than is replaced by the implant (the femoral slideway).The “diminution” of the femoral slideway thus brought about in thedorsal region, in comparison to the original dimensions of the(resected) condyles or to a femoral slideway fitted in the conventionalmanner, produces an effective reduction of the turning radius of thetibial plate belonging to the prosthesis system and hence reduces theload on the collateral ligaments.

The decrease in the dimensions of the femoral slideway measured betweenthe outermost, dorsoventrally opposed points on the condyle shellsurfaces, in comparison to the previously customary dimensioning, ispreferably in the range between 2 and 5%. This is achieved byconstructing the associated femoral slideway/femur-size templatearrangement in such a way that the distance separating one or more pegson the femoral slideway from its dorsal sliding surface is smaller by5-15%, in particular by about 10%, than the corresponding distance bywhich bores in the femur-size template for positioning the pegs areseparated from the contact surface that is to be apposed to the dorsalcondyle surfaces of the femur.

The distance between the dorsal sliding surfaces and the one or morepegs on the inside of the femoral slideway is preferably in the rangebetween 24 and 34 mm and in particular is 29 mm, the chosen valueadvantageously being kept constant in a knee endoprosthesis system forcovering a relevant joint-size range.

As noted above, the femoral slideway/femur-size template arrangement, isconstructed in such a way that the distance between one or more pegs onthe femoral slideway and its dorsal sliding surface is smaller than thecorresponding distance between bores in the femur-size template and thecontact surface of the template by 5-15%, and in particular about 10%.Stated differently, the corresponding distance between bores in thefemur-size template and the contact surface of the template is larger by5-15%, an in particular about 10%, than the distance between one or morepegs on the femoral slideway and its dorsal sliding surface.Accordingly, the distance between the bores in the femur-size templateand the contact surface of the template can be between 26.4 mm and 37.4mm (i.e., 10% larger than the range of 24 mm to 34 mm between the dorsalsliding surface and the one or more pegs of the femoral slideway). Inanother embodiment, the distance between the bores in the femur-sizetemplate and the contact surface of the template can be about 32 mm(i.e., about 10% larger than a distance of 29 mm between the dorsalsliding surface and the one or more pegs of the femoral slideway). Inyet another embodiment, the distance between the bores in the femur-sizetemplate and the contact surface of the template can be between 30.45 mmand 33.35 mm (i.e., 5-15% larger than a distance of 29 mm between thedorsal sliding surface and the one or more pegs of the femoralslideway).

Another distinguishing feature of the proposed femoral slideway is thatparticular dimensions maintain a largely constant relationship to oneanother, regardless of the size of the actual prosthesis. For instance,the ratio a:c between the maximal dorsoventral extent and the maximallateral extent of the femoral slideway is about 0.9±0.02. The patellarpit formed between the condyle shells preferably has a depth “b”,measured from the dorsalmost point on the condyle shells, such that itsratio b:a to the maximal dorsoventral extent of the femoral slideway isin the range between 0.4 and 0.5, in particular is 0.44.

The patellar pit is thus lengthened in the dorsal direction, as a resultof which the patella can be supported over a large area throughout itsentire functional range of flexion.

This elongation of the patellar pit, which furthermore increases inaccordance with the anatomy in implants of all sizes, allows for thefact that the patello-femoral contact surface in conventional femoralslideways has a relatively small bearing area. That is, in the region inwhich the patella leaves the trochlea and enters the intercondylarfossa, conventional femur components provide support only in theperipheral regions.

Furthermore, in the proposed femoral slideway the condyle shells aresomewhat more strongly rounded in cross section (coronal section) thanis the case in conventional femoral slideways. This modification wasundertaken in the interest of improving the fit to the special tibiainsert that belongs to a knee endoprosthesis system, but which is notwithin the scope of the invention.

The back surface of the femoral slideway, in one advantageousembodiment, bears a two-component Ti coating produced in a vacuum plasmaprocedure, consisting of a relatively thin, dense base layer and aseveral fold thicker, open-pored cover layer. The dense base layerallows the femoral slideway, which for example consists of CoCrMo, tobecome completely sealed to the bone and because it makes contact withthe substrate over a large area, increases the stability of adhesion.

The open-pored and very rough surface of the cover layer provides idealconditions for the growth of bony substance onto and into the carriage,producing a quasi “3-D interlocking” that can transmit pulling forces aswell as pressure and transverse forces.

In accordance with one embodiment, a system for sizing and installing afemoral slideway implant is provided. The system comprises a femur sizetemplate comprising a base plate having at least one hole, at least onecondyle engaging surface extending from the base plate, and ameasurement tongue slidably mounted to the base plate, the tongue beingmovable between a plurality of discrete positions relative to thecondyle engaging surface, Wherein a first dimension is defined by adistance between the hole of the base plate and the condyle engagingsurface. The system also comprises a femoral slideway implant comprisingat least one condyle shell having an outer surface defining a dorsalsliding surface, a patellar shield located anterior to the condyleshell, and at least one peg on an inner surface of the implant andbetween the patellar shield and the condyle shell wherein a seconddimension is defined by a distance along a line extendingperpendicularly between a plane tangent to the dorsal sliding surfaceand a longitudinal axis of the peg. The first dimension of the sizetemplate is larger than the second dimension of the implant by apredetermined amount.

In accordance with another embodiment a system, for installing a femoralslideway implant is provided. The system comprises a femur size templatecomprising at least one condyle engaging surface and a tongue configuredto measure a maximal anterior posterior extent of the head of the femur.The system also comprises a femoral slideway implant comprising at leastone condyle shell having an outer surface defining a dorsal slidingsurface, and a patellar shield located anterior to the condyle shell,wherein a maximal anterior posterior extent of the condyle shell is lessthan the maximal anterior posterior extent of the femur.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and useful features of the invention will beapparent from the subordinate claims and the following description of anexemplary embodiment with reference to the figures, wherein

FIG. 1 is a view (from proximal) of a femoral slideway according to oneembodiment of the invention,

FIG. 2 shows the femoral slideway according to FIG. 1 in median section(sagittal section),

FIG. 3 is a plan view of an embodiment of a femur size template,

FIG. 4 is a side view of the latter,

FIGS. 5a, 5b show a conventional arrangement of a femoral slideway on afemur in comparison to an arrangement proposed here, and

FIGS. 6a, 6b show scanning electron micrographs of a cross section of aconventional layered structure and of an embodiment of the layeredstructure proposed here for the back-surface coating of a femoralslideway.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2 the femur component 10, called a femoral slideway, of aknee endoprosthesis is shown. The femoral slideway 10 comprises twoconvexly curved condyle shells 11, 12 and a patellar shield 13, whichconnects the two condyle shells 11, 12 rigidly to one another.

The condyle shells 11, 12 and the patellar shield 13 in their interiorsdefine anterior and posterior fitting surfaces 14, 15 that correspond toa femoral ventral, and dorsal cut, respectively, and are associated witha ventral and a dorsal saw-cut surface produced at the distal end of thefemur when the latter was resected for fitting of the femoral slideway.The convex outer shape of the condyle shells 11, 12 specifies dorsalsliding surfaces 11 a, 12 a in the posterior region, over which thecorresponding surfaces of the tibia insert slide when the kneeendoprosthesis is flexed. The patellar shield 13, which is recessed withrespect to the convex outer surfaces of the condyle shells 11, 12,defines a so-called patellar pit 16, within which there is supported apatella component 17 of the knee endoprosthesis, which is indicated by adashed outline in FIG. 2 and does not belong to the femoral slideway 10.

To assist anchoring and central placement of the femoral slideway 10 onthe femur, on the inner surface of the femoral slideway two pegs 18, 19are formed, the long axis of which is substantially parallel to theposterior fitting surface 15. These pegs project into holes in thefemur, which have been drilled in the appropriate positions with the aidof a corresponding drilling template (see below), and this engagementgives the attachment of femoral slideway to bone greater stability thanis provided by the fitting surfaces alone.

To ensure that the femoral slideway will function optimally as areplacement for destroyed sliding surfaces on the femur, theconstruction must reflect as accurately as possible the anatomicalarrangements and dimensions, but also within the scope of the inventionincludes a specific modification that will now be explained.

One of the relevant dimensions of the femoral slideway is the maximalanterior-posterior or dorsoventral extent of the condyle shells 11, 12,a distance labeled “a” in FIG. 1. Another relevant dimension is themaximal lateral extent of the femoral slideway, i.e., the distancebetween the most lateral point on the lateral condyle shell 11 and themost medial point on the medial condyle shell 12, which in FIG. 1 islabeled “c”. Also significant is the distance from the outermostposterior point on the dorsal sliding surfaces 11 a, 12 a of the condyleshells 11, 12 to the posterior bounding edge of the patellar shield 13,which in FIG. 1 is labeled “b”. A final significant distance is thatbetween the outermost posterior points on the dorsal sliding surfaces 11a, 12 a and the long axis of the pegs 18, 19 (which lie in one and thesame coronal plane), labeled “d” in FIG. 2. In the exemplary embodimentdescribed here, the ratio a:c is 0.9 and the ratio b:a is 44. On groundsof biomechanics and surgical technique, it has proved useful to make thedistance “d” (between sliding surface and peg axis) uniform for allsizes of femoral slideways used in a knee endoprosthesis system. In thepresent case, this distance is 29 mm.

To determine the correct femoral slideway size, a femur-size template 20shown in FIGS. 3 and 4 is used. This comprises a basic part 21 with twoflanks 22 and 23, each of which ends in a contact section 22 a, 23 athat is bent at a right angle and is apposed to the condyles of afemoral bone that is to be fitted with a femoral slideway (FIGS. 1 and2).

In the middle of the basic part 21 a measurement tongue 24, bent at anangle in two places, is mounted so that it can be displaced in adirection perpendicular to the plane in which the contact sections 22 a,23 a lie. The measurement tongue 24 is marked with a scale 25, whichindicates the maximal anterior-posterior extent of the head of thefemur, i.e. the condyles, and thus indicates to the doctor the requiredsize of the implant. In the basic part 21 of the femur-size template 20two peg-hole bores 26, 27 are provided, which—in accordance with asupplementary drilling-template function of the femur-sizetemplate—assist the positioning of peg-holes in the femur so that theycorrespond to the pegs 18, 19 of the femoral slideway 10, as shown inFIG. 1. The axes of the peg-hole bores 26, 27 are separated by adistance “e” from the contact surfaces of the contact sections 22 a, 23a.

This distance—along with the distance “d” between the sliding surfacesand pegs on the femoral slideway 10 itself (cf. FIG. 2)—is an additionalrelevant dimension in the concrete implementation of a kneeendoprosthesis, for the following reason:

So that the above-mentioned peg-holes—which serve not only to positionthe implant but also to position the cutting guides used to produce thevarious saw cuts on the femur—can be drilled into the bone, drill bushes(not shown) are inserted into the peg-hole bores 26, 27.

It has proved advantageous, in particular from the viewpoint of reducingthe load on the collateral ligaments during flexion of the artificialknee joint, to resect more bone dorsally on the femur than will bereplaced there by the thickness of the dorsal parts of the condyleshells. For this reason the distance “e” is made larger than thecorresponding distance “d” (FIG. 2). In the preferred embodiment, therelative distance reduction, i.e. the quantity (e−d)/d, is about 10%.

The effect thus achieved can be seen in FIG. 5, where a sketchrepresenting the conventional way of attaching a femoral slideway 10′ toa femur F′, shown in FIG. 5a , is compared with the representation inFIG. 5b of the arrangement proposed here. The anterior-posterior extentof the femoral slideway 10 in FIG. 5b , mounted on a femur F resectedfurther in the dorsal region, is smaller by the amount (e−d) than in theconventional implant 10′.

Because the distance “e” is permanently specified by the femur-sizetemplate, which is used for ail implants regardless of their size, andaccording to what has been stated above, the distance “d” in theembodiment of the femoral slideway is preferably kept constant for allimplant sizes, the geometric relations will be slightly different forimplants of different sizes. This is acceptable, however, in view of theadvantages for manufacture and manipulation that such a system brings.

FIG. 6b shows—in comparison to a conventional femur carriage coating asshown in FIG. 6a —the appearance in the scanning electron microscope ofa cross-section through a two-component titanium-coating constructionconsisting of a dense base layer G, about 50 μm thick, and an open-poredcover layer D averaging about 250 μm thick, on a CoCrMo substrate S.Although the thickness and average roughness of the coating according toFIG. 6b , which is applied by a vacuum plasma process, are comparable tothose of the known, sprayed-on coating according to FIG. 6a , it shouldbe emphasized that the former has a more open-pored structure and aconsiderably reduced number of interface defects (indicated in bothpictures by vertical arrows).

Implementation of the invention is not limited to the exemplaryembodiment described above, but can also incorporate modifications,which in particular include departures from the specified dimensions andratio values.

LIST OF REFERENCE NUMERALS

-   10, 10′ Femoral slideway-   11, 12 Condyle shells-   11 a, 12 a Dorsal sliding surfaces-   13 Patellar shield-   14 Anterior fitting surface-   15 Posterior fitting surface-   16 Patellar pit-   17 Patella component-   18, 19 Peg-   20 Femur-size template-   21 Basic part-   22, 23 Flanks-   22 a, 23 a Contact sections-   24 Measurement tongue-   25 Scale markings-   26, 27 Peg-hole bores-   a, b, c, d, e Distances-   A-A Plane of section-   D Cover layer-   F, F′ Femur (shaped)-   G Base layer-   S Substrate

What is claimed is:
 1. A method for implanting a femoral slideway on afemur, comprising: locating a point on a lower extremity of a femur,said point being located at a predetermined distance from a planetangent to at least one dorsalmost point of lateral and medial condylesof the femur, the pre-determined distance defined by a template having apermanently specified distance between a bore on a support part and acontact surface on a flank of the template, the support part and flankdefined as a single piece, the contact surface configured to engage atleast one of the dorsalmost points of the lateral and medial condyles ofthe femur; drilling a hole at said point; and selecting a femoralslideway having a dimension defined by a distance between a pegextending from the slideway, the peg configured for insertion into acorresponding hole formed in the femur, and a plane tangent to a dorsalsliding surface furthest away from the peg; and wherein saidpre-determined distance is larger than said dimension by an amountdetermined prior to the locating step, and wherein locating said pointincludes engaging the contact surface with the at least one of thedorsalmost points of the lateral and medial condyles of the femur toalign said bore with said point.
 2. The method of claim 1, wherein saidpre-determined distance is about 5% to about 15% larger than saiddimension of the femoral slideway.
 3. The method of claim 1, whereinselecting further comprises selecting a femoral slideway from a group ofslideways, all of the slideways in said group having substantially equalpeg-to-sliding-surface dimensions as defined by a distance between thepeg and the plane tangent to the dorsal sliding surface furthest awayfrom the peg, wherein each slideway in said group has a different dorsalcondyle-to-ventral condyle distance.
 4. The method of claim 1, furthercomprising resecting bone material from the femur such that more bonematerial is removed from a dorsal side of the femur than will bereplaced by the slideway to allow for a reduction of the distancebetween the peg and a center of a turning radius of the dorsal side ofthe slideway, as compared to the unresected femur, in order to reduce aload on ligaments attached to the femur.
 5. The method of claim 4,wherein the resecting includes removing about 10% more bone materialfrom the dorsal side of the femur than is replaced by the slideway, asmeasured between said hole and a dorsalmost surface of the femur.
 6. Themethod of claim 1, wherein said pre-determined distance is at least 5%larger than said dimension of the femoral slideway, and wherein saidpre-determined distance is no greater than 15% larger than saiddimension of the femoral slideway.
 7. The method of claim 6, whereinsaid pre-determined distance is at least 10% larger than said dimensionof the femoral slideway.
 8. The method of claim 1, wherein saidpre-determined distance corresponds to the permanently specifieddistance of the template irrespective of the size of the femoralslideway.
 9. The method of claim 1, wherein the femoral slidewaycomprises a plurality of femoral slideways of different sizes, andwherein said dimension is a constant distance for the plurality offemoral slideways.
 10. A method for implanting a femoral slideway on afemur, comprising: locating a point on a lower extremity of a femur,said point being located at a predetermined distance from a planetangent to at least one dorsalmost point of lateral and medial condylesof the femur, the pre-determined distance defined by a template having apermanently specified distance between a bore on a support part and acontact surface on a flank of the template, the support part and flankdefined as a single piece, the contact surface configured to engage atleast one of the dorsalmost points of the lateral and medial condyles ofthe femur; drilling a hole at said located point; resecting apredetermined amount of bone between the located point and the planetangent to the at least one dorsalmost point; and selecting a femoralslideway having a peg extending from the slideway and at least onecondyle shell, the peg configured for insertion into a correspondinghole formed in the femur, said condyle shell having a thickness lessthan the predetermined amount of resected bone; and wherein locatingsaid point includes engaging the contact surface with the at least oneof the dorsalmost points of the lateral and medial condyles of the femurto align said bore with said point.
 11. The method of claim 10, whereinthe condyle shell comprises a dorsal sliding surface and a posteriorfitting surface, the thickness of the condyle shell being definedbetween said surfaces.
 12. The method of claim 10, wherein selectingfurther comprises selecting a femoral slideway from a group ofslideways, all of the slideways in said group having substantially equalpeg-to-sliding-surface dimensions as defined by a distance between thepeg and a plane tangent to a dorsal sliding surface of the condyle shellfurthest away from the peg, wherein each slideway in said group has adifferent dorsal condyle-to-ventral condyle distance.
 13. The method ofclaim 10, wherein the thickness of the condyle shell is alsopredetermined.
 14. The method of claim 10, wherein the resecting allowsfor a reduction of the distance between the peg and a center of aturning radius of the dorsal side of the slideway, as compared to theunresected femur, in order to reduce a load on ligaments attached to thefemur.
 15. The method of claim 10, wherein the resecting includesremoving about 10% more bone material from the dorsal side of the femurthan is replaced by the femoral slideway, as measured between said holeand a dorsalmost surface of the femur.
 16. The method of claim 10,wherein the femoral slideway comprises a plurality of femoral slidewaysof different sizes.